[R] How to decrease size of points?

[Medic]

The code works as I want, but the points (circles) on the plot are too
big. How to decrease them? Where to insert (for instance) size = 0.8
for points (circles) on plot?
p1 <- p + geom_point(aes(size = Stage), alpha = 1/3) + xlab ("X") +
ylab("Y") + geom_smooth()
Stage is factor, x and y - continuous

<Rui Barradas: add the scale_size
p1 + scale_size_manual(values = 0.8)>

Thanks Rui, but I got:
Error: Insufficient values in manual scale. 12 needed but only 1 provided.
(or Error: Continuous value supplied to discrete scale)